Condition Factsheets

Placenta Previa

The Facts on Placenta Previa

The placenta is a bed of cells formed inside the uterus (womb) during pregnancy. The purpose of the placenta is to carry nourishment and oxygen from the mother to the fetus and to carry waste products and carbon dioxide from the fetus to the mother through the umbilical cord.

The placenta is usually formed along the upper part of the uterus, allowing enough space for the fetus to grow. In placenta previa, the placenta starts forming very low in the uterus or even over the cervix (the opening of the uterus that leads to the vagina). This obstruction impairs normal vaginal delivery of the baby at birth.

There are 3 types of placenta previa:

complete placenta previa: The internal cervical opening is completely covered by the placenta.

partial placenta previa: The internal cervical opening is partially covered by the placenta.

marginal placenta previa: The placenta is at the edge of the internal cervical opening.

Placenta previa is estimated to occur in 1 in 200 pregnancies.

Causes of Placenta Previa

The causes and risk factors for placenta previa are:

The fertilized egg implanted very low in the uterus, causing the placenta to form close to or over the cervical opening.

The lining of the uterus (endometrium) has abnormalities such as fibroids or scarring (from previous previa, incisions, caesarean sections or abortions).

The placenta formed abnormally.

The pregnancy is multiple (i.e., twins or triplets). The chances of developing placenta previa are doubled for these pregnancies.

The mother may have had several previous pregnancies. The chances of developing placenta previa are increased to 1 in 20 for women who have had 6 or more pregnancies.

The mother smokes or uses cocaine. Smoking and cocaine use can increase the risk of this condition.

The mother is older. The risk of developing placenta previa is 3 times as high in women over 30 years of age as in women under 20 years of age.

The pregnancy has been conceived with the help of assisted reproductive technology, such as in-vitro fertilization.

Symptoms and Complications of Placenta Previa

Slight and occasional bleeding may occur during the first and second trimester of pregnancy. The color of the blood may be bright red and it may start and stop, then restart after several days or weeks. Sudden and excessive bleeding may occur in the third trimester of pregnancy.

Bleeding is usually not accompanied by pain, although uterine cramping may occur at the time of bleeding in some women. In 7% to 30% of women there may be no bleeding at all.

The bleeding occurs because as the pregnancy progresses, the placenta gets detached from the uterine walls. In the third trimester the uterine walls become thinner and spread to accommodate the growing fetus. If placenta previa is present, the placenta is attached very low on the uterine wall. This thinning makes the placenta stretch and tear away from the uterine wall, leading to bleeding.

The following are the possible complications of placenta previa:

slow fetal growth due to insufficient blood supply

fetal anemia

fetal distress due to low oxygen supply

shock and death of the mother if the bleeding is excessive

infection and formation of blood clots

blood loss requiring transfusion

If you are diagnosed with complete placenta previa, your doctor may advise you to have a caesarean section for the delivery of your baby.

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